Geochemical Evidence of a Metasomatized Mantle Source Beneath the West Antarctic Rift System

We analyzed alkaline samples dredged from seamounts in the western Ross Sea, Antarctica, to determine their trace element concentrations and Pb, Sr, Nd and Hf isotopic compositions aimed at characterizing source components and melting processes. These samples yield very narrow ranges in isotopic ratios (206Pb/204Pb = 19.279125-19.98534, 207Pb/204Pb = 15.475681 - 15.629633, 87Sr/86Sr = 0.702885 - 0.703524, 143Nd/144Nd = 0.512883 - 0.513010 176Hf/177Hf = 0.282977 - 0.283061), suggesting a homogenous mantle source. This is consistent with previous studies of subaerial volcanism from the West Antarctic Rift System (WARS) and the associated diffuse alkaline magmatic province of the southwest Pacific. The large geographic extent of this homogeneous Cenozoic volcanism supports a passive rifting model and resultant decompressional melting (e.g., Wörner, 1999), rather than the mantle plume model supported by high average elevations for the Antarctic continent and the HIMU isotopic signature observed throughout the WARS (e.g., Behrendt, 1999). Large low-velocity seismic anomalies have been observed at 50-200 km beneath the WARS (Finn et al., 2005) and may represent a "wet" mantle modified by subduction along the paleo-Pacific margin of Gondwana. During this 400-Myr subduction history, infiltration of subduction related melts from the downgoing slab must have metasomatized the mantle, with concomitant partial melting generating pyroxenite dikes and veins in the overlying mantle wedge. The lower solidus of these pyroxenites allows generation of higher melt volumes relative to a dry peridotite source under the same conditions. Similarities between major element concentrations (e.g., high TiO2, Na 2O) and trace element ratios (e.g., Nb/Th) of these new samples from the Ross Sea with experimental data for garnet- bearing pyroxenite melts (Kogiso et al., 2003, 2004) further support a metasomatized mantle source for WARS magmatism.